<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Dependency Tracking</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.1.2 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Data Definition"
HREF="ddl.html"><LINK
REL="PREVIOUS"
TITLE="Other Database Objects"
HREF="ddl-others.html"><LINK
REL="NEXT"
TITLE="Data Manipulation"
HREF="dml.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2011-12-01T22:07:59"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.1.2 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Other Database Objects"
HREF="ddl-others.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="ddl.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 5. Data Definition</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Data Manipulation"
HREF="dml.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="DDL-DEPEND"
>5.12. Dependency Tracking</A
></H1
><P
>   When you create complex database structures involving many tables
   with foreign key constraints, views, triggers, functions, etc. you
   implicitly create a net of dependencies between the objects.
   For instance, a table with a foreign key constraint depends on the
   table it references.
  </P
><P
>   To ensure the integrity of the entire database structure,
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> makes sure that you cannot
   drop objects that other objects still depend on.  For example,
   attempting to drop the products table we had considered in <A
HREF="ddl-constraints.html#DDL-CONSTRAINTS-FK"
>Section 5.3.5</A
>, with the orders table depending on
   it, would result in an error message such as this:
</P><PRE
CLASS="SCREEN"
>DROP TABLE products;

NOTICE:  constraint orders_product_no_fkey on table orders depends on table products
ERROR:  cannot drop table products because other objects depend on it
HINT:  Use DROP ... CASCADE to drop the dependent objects too.</PRE
><P>
   The error message contains a useful hint: if you do not want to
   bother deleting all the dependent objects individually, you can run:
</P><PRE
CLASS="SCREEN"
>DROP TABLE products CASCADE;</PRE
><P>
   and all the dependent objects will be removed.  In this case, it
   doesn't remove the orders table, it only removes the foreign key
   constraint.  (If you want to check what <TT
CLASS="COMMAND"
>DROP ... CASCADE</TT
> will do,
   run <TT
CLASS="COMMAND"
>DROP</TT
> without <TT
CLASS="LITERAL"
>CASCADE</TT
> and read the <TT
CLASS="LITERAL"
>NOTICE</TT
> messages.)
  </P
><P
>   All drop commands in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> support
   specifying <TT
CLASS="LITERAL"
>CASCADE</TT
>.  Of course, the nature of
   the possible dependencies varies with the type of the object.  You
   can also write <TT
CLASS="LITERAL"
>RESTRICT</TT
> instead of
   <TT
CLASS="LITERAL"
>CASCADE</TT
> to get the default behavior, which is to
   prevent the dropping of objects that other objects depend on.
  </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>    According to the SQL standard, specifying either
    <TT
CLASS="LITERAL"
>RESTRICT</TT
> or <TT
CLASS="LITERAL"
>CASCADE</TT
> is
    required.  No database system actually enforces that rule, but
    whether the default behavior is <TT
CLASS="LITERAL"
>RESTRICT</TT
> or
    <TT
CLASS="LITERAL"
>CASCADE</TT
> varies across systems.
   </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>    Foreign key constraint dependencies and serial column dependencies
    from <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> versions prior to 7.3
    are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> maintained or created during the
    upgrade process.  All other dependency types will be properly
    created during an upgrade from a pre-7.3 database.
   </P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="ddl-others.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="dml.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Other Database Objects</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="ddl.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Data Manipulation</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>